1. Field of the Invention
The present invention relates to a semi-conductive roll such as a developing roll, for use in office automation (OA) machines or devices such as electrophotographic copying machines, printers, and telecopiers.
2. Discussion of Related Art
Semi-conductive rolls such as a developing roll and a charging roll are installed on office automation (OA) machines or devices such as electrophotographic copying machines, printers, and telecopiers. For instance, the developing roll is installed such that it is in contact with the toner, so that an electrostatic latent image formed on an outer circumferential surface of a photosensitive drum as an image bearing medium is developed into a visible image. The charging roll is installed on the machines such that the charging roll is rotated while it is held in contact with the photosensitive drum. Thus, the semi-conductive rolls perform respective functions.
Described more specifically, the developing roll carries a layer of toner on its outer circumferential surface. The developing roll and the photosensitive drum are rotated while the developing roll is held in contact with the photosensitive drum on which the latent image is formed, so that the latent image is developed into a toner image. The charging roll and the photosensitive drum are rotated such that the charging roll to which a voltage is applied is held in pressing contact with the outer circumferential surface of the photosensitive drum, to thereby charge the outer circumferential surface of the photosensitive drum.
Such semi-conductive rolls described above include a suitable shaft (metal core) as an electrically conductive body and an electrically conductive base layer with a suitable thickness formed on an outer circumferential surface of the shaft and constituted by a solid elastic body, a foamed elastic body or the like. The semi-conductive rolls further include, as needed, an intermediate layer and a surface layer in the form of a resistance adjusting layer and a protective layer formed radially outwardly of the base layer, for the purpose of adjusting the electric resistance of the roll and protecting the base layer having a relatively low hardness.
In recent years, there have been increasing demands for high image quality and energy saving (reduction of electric power consumption) in the office automation (OA) machines or devices such as the copying machines, printers, and telecopiers. To meet such demands, in place of a conventionally employed crushed toner, there is employed spherical polymeric toner having a relatively small particle size and particle size difference and a low melting point, so that the toner particles can be uniformly charged.
Where a pressure of contact between the semi-conductive roll and the photosensitive drum is relatively large, the polymeric toner having a lowered melting point tends to be broken or deformed by softening, and the particles of the toner tend to aggregate, making it difficult to attain the intended high image quality and energy saving. In view of this, the semi-conductive roll needs to be arranged so as to assure careful handling of the toner to prevent a large stress acting on the toner. To this end, the hardness of the base layer which influences the hardness of the roll is lowered. Further, the intermediate layer and the surface layer are formed of a soft rubber material or an elastomer material in view of a fact that the roll tends to suffer from creases or wrinkles if a difference between the hardness of the base layer and the hardness of the intermediate or surface layer formed radially outwardly of the base layer increases.
Where the intermediate layer or the surface layer is formed by using the rubber material or the elastomer material according to a known coating method such as dipping or roll coating on the low-hardness base layer, in particular on the low-hardness base layer constituted by a solid elastic body, the intermediate layer or the surface layer serving as the coating layer does not have a sufficient crosslinking density, so that the roll may not exhibit a wear resistance high enough to withstand a long period of use. In addition, the coating layers of individual rolls have different thickness values due to a progress of scorching of the rubber component in the coating liquid. In this case, the rolls do not have an intended surface condition required to attain the high image quality. If the amount of the crosslinking agent to be added to the coating liquid is decreased in order to permit the coating liquid to be stored at room temperature with high stability without suffering from the scorching, the crosslinking or vulcanization does not proceed, undesirably increasing a time period required for the vulcanization and deteriorating the production efficiency. In addition, the crosslinking density of the coating layer is undesirably lowered.
In general, since the amount of the coating liquid to be prepared for the coating operation for forming the intermediate layer or the surface layer is larger than that actually used in the coating operation, a part of the coating liquid is inevitably left unused. The unused coating liquid is recovered and recycled in view of the cost. In the recycling process, the scorching of the rubber component in the coating liquid progresses, so that the coating liquid tends to be gelled, producing agglomerates. If the coating liquid which includes the agglomerates is coated on the outer surface of the base layer, the roll undesirably suffers from surface defects, increasing the reject ratio.
Conventionally, the surface of the semi-conductive roll, in particular the surface of the developing roll is slightly roughened for improving its toner transferring property. For instance, the surface of the base layer is suitably roughened by grinding or molding, so that the roll has a desired surface roughness. Alternatively, as disclosed in JP-A-2000-330372, a roughening agent such as a spherical filler is added to the coating layer (serving as the intermediate layer or the surface layer), so that the roll has a desired surface roughness. Owing to the use of the polymeric toner described above, the uniform charging of the toner is realized for attaining high image quality. To attain further improved image quality, it is required to precisely control the surface roughness of the roll. Where the intermediate layer or the surface layer of the roll is formed by the coating operation, however, the coating layers of individual rolls undesirably have different thickness values, making it quite difficult to control the surface roughness as desired.